US3146402A - Frequency-modulated subcarrier detector - Google Patents

Frequency-modulated subcarrier detector Download PDF

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Publication number
US3146402A
US3146402A US84636A US8463661A US3146402A US 3146402 A US3146402 A US 3146402A US 84636 A US84636 A US 84636A US 8463661 A US8463661 A US 8463661A US 3146402 A US3146402 A US 3146402A
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US
United States
Prior art keywords
pulses
signal
subcarrier
frequency
series
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US84636A
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English (en)
Inventor
Bernard D Loughlin
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Hazeltine Research Inc
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Hazeltine Research Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to NL133956D priority Critical patent/NL133956C/xx
Priority to NL273844D priority patent/NL273844A/xx
Application filed by Hazeltine Research Inc filed Critical Hazeltine Research Inc
Priority to US84636A priority patent/US3146402A/en
Priority to SE12296/61A priority patent/SE305667B/xx
Priority to GB45539/61A priority patent/GB945268A/en
Priority to CH1482561A priority patent/CH393449A/de
Priority to DES77304A priority patent/DE1195825B/de
Priority to FR885826A priority patent/FR1312325A/fr
Application granted granted Critical
Publication of US3146402A publication Critical patent/US3146402A/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04H—BROADCAST COMMUNICATION
    • H04H20/00—Arrangements for broadcast or for distribution combined with broadcast
    • H04H20/86—Arrangements characterised by the broadcast information itself
    • H04H20/88—Stereophonic broadcast systems
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03D—DEMODULATION OR TRANSFERENCE OF MODULATION FROM ONE CARRIER TO ANOTHER
    • H03D3/00—Demodulation of angle-, frequency- or phase- modulated oscillations
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03D—DEMODULATION OR TRANSFERENCE OF MODULATION FROM ONE CARRIER TO ANOTHER
    • H03D3/00—Demodulation of angle-, frequency- or phase- modulated oscillations
    • H03D3/02—Demodulation of angle-, frequency- or phase- modulated oscillations by detecting phase difference between two signals obtained from input signal
    • H03D3/04—Demodulation of angle-, frequency- or phase- modulated oscillations by detecting phase difference between two signals obtained from input signal by counting or integrating cycles of oscillations
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B14/00—Transmission systems not characterised by the medium used for transmission
    • H04B14/08—Transmission systems not characterised by the medium used for transmission characterised by the use of a sub-carrier
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J1/00—Frequency-division multiplex systems
    • H04J1/20—Frequency-division multiplex systems in which at least one carrier is angle-modulated

Definitions

  • the frequency-modulated multiplex signal mentioned above may be an SCA (Subsidiary Communications Authorization) type signal wherein a subcarrier in the order of 50 kilocycles is frequency-modulated by a subcarrier modulating signal.
  • the main carrier in the order of 100 megacycles and representative of an FM broadcast channel frequency, is frequency-modulated by both a main modulating signal and the frequency-modulated subcarrier signal.
  • two modulating signals are transmitted in one channel and on one main carrier which is representative of the channel frequency.
  • the main modulating signal and subcarrier are detected and separated and the subcarrier modulating signal, in turn, is detected from the subcarrier.
  • the main modulating signal may be the usual program material and the subcarrier modulating signal may be the familiar subscription type material heard as background music in restaurants, stores, and oflices.
  • this multiplex technique may be used for stereo broadcasting.
  • the conventional technique of detecting the modulation of the frequency-modulated subcarrier signal with a counter-type detector is to amplify and limit the sub carrier signal and to then differentiate the limited signal.
  • either the positive or negative differentiated pulses are counted or integrated in an integrating circuit to derive an audio-frequency signal having an amplitude which varies in proportion to changes in the repetitionfrequency of the pulses which are counted and which is, therefore, representative of the modulation of the frequency-modulated subcarrier signal.
  • the amplitude of this audio-frequency signal naturally is dependent upon the area under or the energy of the ditferentiated pulses which are counted.
  • any attempt to increase the ampli tude of the audio-frequency signal by merely increasing the time constant of the differentiating circuit to derive differentiated pulses having longer durations and higher average amplitudes results in degradation of the linearity of the detector since at high frequencies the differentiated pulses will have insuflicient time to decay to approxibately zero before the occurrence of the next differentiated pulse.
  • the amplitude variations of this audio-frequency signal be dependent only upon the changes in repetition frequency of the pulses counted. Variations in amplitude due to changes in the energy content of the pulses which are counted result in loss of linearity; the energy content of the pulses which are counted must be constant.
  • the present invention is directed to a detector Wherein these pulses which are counted are formed or shaped to have a greater area or energy content than the energy content of conventionally derived differentiated pulses and when counted these pulses do not cause the linearity of the detector to be degraded.
  • This provides an audiofrequency signal at the output of the detector which has a greater amplitude than one derived by conventional techniques and this detector is said to have an improved audio recovery.
  • apparatus for measuring the repetition rate of a repetitive signal comprises means for amplitude-limiting the repetitive signal and differentiating means for deriving from the amplitude limited signal a series of pulses.
  • the apparatus additionally includes means for shaping each pulse in the series whereby each pulse so shaped has a higher energy content than corresponding ones of the series derived by the differentiating means.
  • the apparatus also includes means for rectifying the shaped series of pulses retaining a series of same polarity pulses which could be obtained from the series of pulses derived by the differentiating means whereby the output of the apparatus is substantially enhanced.
  • FIG. 1 is a circuit diagram, partly schematic, representing a complete frequency-modulation carrier signal receiver along wtih a subcarrier unit which includes apparatus for detecting the modulation of a frequency-modulated subcarrier signal constructed in accordance with the present invention
  • FIGS. 2a and 2b show voltage wave forms useful in understanding the advantages of the detector apparatus of the present invention.
  • FIG. 1 there is represented a complete frequency-modulated carrier signal receiver 19 of conventional construction along with a subcarrier unit which includes apparatus for detecting the modulation of a frequency-modulated subcarrier signal constructed in accordance with the present invention.
  • the receiver 10 may include the usual circuits normally found in such a device.
  • the receiver 10, having its input terminal connected to an antenna 11, may include a radio-frequency amplifier, an oscillator-modulator, an intermediate-frequency amplifier, a frequency-modulation detector, and an audio-frequency amplifier, all of conventional construction for deriving, in the usual manner, an audio-frequency signal representing the main modulating signal.
  • Such a signal may be reproduced, in a conventional manner, by a sound reproducer 12.
  • a subcarrier unit composed of a buffer amplifier 13, a bandpass filter 14, a subcarrier amplifier 15, a subcarrier limiter 16, a subcarrier detector 17 shown within dotted lines and more fully described hereinafter, an audio-frequency amplifier 18, and a sound reproducer 19. All the elements in the subcarrier unit except for the subcarrier detector 17 may be of conventional construction and operate in the usual manner.
  • the input circuit of the buffer amplifier 13 is connected to the detector output, before de-emphasis, of the receiver 10.
  • the buffer amplifier 13 serves to isolate the relatively low impedance of the bandpass filter 14 from the relatively high impedance of the detector in the receiver 10.
  • the buffer amplifier .13 should be designed so that its distortion is minimized since harmonics of the main modulating signal may fall within the subcarrier pass band and result in crosstalk.
  • the bandpass isease filter 14, having a pass band substantially equal to the maximum frequency deviation of the subcarrier signal and centered approximately at the subcarrier frequency passes the subcarrier signal and further attenuates the main channel audio-frequency signal developed by the detector of the receiver Ill as well as noise signals in the frequency range above and below this pass band.
  • the subcarrier signal is amplified by the subcarrier amplifier and is amplitude limited by the subcarrier limiter 16.
  • the amplitude-limited subcarrier signal is supplied to the subcarrier detector 17, the operation of which will be described more fully hereinafter whereat an audio-frequency signal representative of the modulation of the frequency-modulated subcarrier signal is derived which is, in turn, amplified by the audio-frequency amplifier 18 and reproduced by the sound reproducer 19 in a conventional manner.
  • this detector comprises differentiating means Zfl, including an inductor 21, for developing a series of pulses from the amplitude-limited signal supplied by the subcarrier limiter l6.
  • Differentiating means 20 may also include a capacitor 22 and a resistor 23.
  • the detector 17 also includes a diode 24 for rectifying the series of pulses developed by the differentiating means 20.
  • the detector 17 may include means for deriving a signal representative of the modulation of the frequency-modulated subcarrier signal from the pulses developed by the differentiating means 20 which are retained after rectification by the diode 24.
  • lvfeans 25 may be a conventional integrating circuit composed of a resistor 26 and a capacitor 27.
  • the differentiating means 26 conventionally develops a series of positive and negative differentiated pulses, as indicated by the solid voltage wave form A of FIG. 211, from the amplitude-limited signal supplied by the subcarrier limiter 16.
  • the difference in the operation of the differentiating means 20 is that the pulses thus developed have a slower initial decay and a faster terminal decay than the differentiated pulses developed in the absence of the inductor.
  • the initial decay corresponds to the beginning of the decay, while the terminal decay corresponds to the end of the decay.
  • the diode 24 When the diode 24 is included in the circuit the series of pulses developed by the differentiating means 21 is rectified. For the polarity indicated in FIG. 1, the positive pulses are shorted to ground by the diode 24. Furthermore, the diode 24 clamps the oscillations developed by the differentiating means 20 so that the positive oscillations associated with the negative pulses are effectively removed. Since the positive pulses are shorted to ground by the diode 24, the negative oscillations associated therewith and shown in FIG. 2a are not developed. Therefore, the diode 24 in rectifying the series of pulses developed by the differentiating means 20 retains the negative pulses.
  • the average value of these pulses is representative of the modulation of the frequency modulated subcarrier signal.
  • This average value may be derived from the pulses by the integrating circuit 25 which integrates or counts the pulses and develops an audio-frequency signal across the capacitor 27.
  • the amplitude of this audio-frequency signal varies in accordance with the changes in the repetition frequency of the d pulses or more particularly the modulation of the subcarrier signal.
  • the detector 1'7 constructed in accordance with the present invention has an improved linearity characteristic compared to the linearity characteristic of a conventional counter-type detector. This may be best illustrated by referring to FIGS. 2a and 2b.
  • the differentiated pulses shown by the solid wave form A of FIG. 2b developed from a higher frequency of the amplitude-limited signal than the differentiated pulses shown in FIG. 20, have insufiicient time to decay to approximately Zero before the occurrence of the next differentiated pulse. There is also a reduction in the peak-to-peak amplitude of the differentiated pulses of FiG. 212.
  • the detector 17 makes possible an improved linearity characteristic without any loss in audio recovery.
  • the time constant of the differentiating means 20 may be increased. In this case, however, the linearity of the detector may become slightly degraded since each pulse may not be fully completed before the next pulse is initiated. This is indicated by the dot-dash wave form C of PEG. 2b. If the time constant of the differentiating means 20 is increased only up to the point where the linearity of the detector corresponds to the linearity of a conventional counter-type detector which does not have the inductor 21, the net result is improved audio recovery with no additional degradation in linearity. This is possible since a detector constructed in accordance with the present invention inherently has an improved linearity characteristic compared to the linearity characteristic of a conventional counter-type detector.
  • Inductor 21 millihenries 50 Capacitor 22 micromicrofarads 100 Resistor 23 kilohms 33 Diode 24 1N34AS Resistor 26 akilohms 100 Capacitor 27 micromicrofarads 1500 Subcarrier frequency l ilocycles 35-65 While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.
  • Apparatus for measuring the repetition rate of a repetitive signal comprising:
  • each pulse so shaped has a higher energy content than corresponding ones of said series derived by said differentiating means
  • said shaping means includes:
  • an inductor and in which each pulse so shaped decays to zero before the next pulse is initiated, has a slower initial decay and a faster terminal decay than corresponding ones in said series derived by said differentiating means whereby the linearity of said apparatus relative to the linearity obtained by said apparatus in the absence of said shaping means is substantially improved.
  • Apparatus for measuring the repetition rate of a repetitive signal comprising:
  • each pulse so shaped has a higher energy content than corresponding ones of said series derived by said differentiating means and (B) each pulse decays to zero before the next pulse is initiated, each pulse has a slower initial decay and each pulse has a faster terminal decay than corresponding ones of said series derived by said differentiating means so that the linearity of said apparatus relative to the linearity obtained by said apparatus in the absence of said shaping means is substantially improved; and means for rectifying said shaped series of pulses retaining a series of same polarity pulses whose average value is representative of the modulation of said frequency modulated subcarrier signal and is greater than the average value of the same polarity pulses which could be obtained from the series of pulses derived by said differentiating means, whereby the output of said apparatus is substantially enhanced.

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Power Engineering (AREA)
  • Stereo-Broadcasting Methods (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
  • Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
US84636A 1961-01-24 1961-01-24 Frequency-modulated subcarrier detector Expired - Lifetime US3146402A (en)

Priority Applications (8)

Application Number Priority Date Filing Date Title
NL133956D NL133956C (de) 1961-01-24
NL273844D NL273844A (de) 1961-01-24
US84636A US3146402A (en) 1961-01-24 1961-01-24 Frequency-modulated subcarrier detector
SE12296/61A SE305667B (de) 1961-01-24 1961-12-08
GB45539/61A GB945268A (en) 1961-01-24 1961-12-19 Frequency-modulated subcarrier detector
CH1482561A CH393449A (de) 1961-01-24 1961-12-22 Frequenzdetektor
DES77304A DE1195825B (de) 1961-01-24 1961-12-27 Demodulator fuer frequenzmodulierte elektrische Schwingungen
FR885826A FR1312325A (fr) 1961-01-24 1962-01-24 Détecteur de la fréquence de récurrence d'une oscillation

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US84636A US3146402A (en) 1961-01-24 1961-01-24 Frequency-modulated subcarrier detector

Publications (1)

Publication Number Publication Date
US3146402A true US3146402A (en) 1964-08-25

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US84636A Expired - Lifetime US3146402A (en) 1961-01-24 1961-01-24 Frequency-modulated subcarrier detector

Country Status (7)

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US (1) US3146402A (de)
CH (1) CH393449A (de)
DE (1) DE1195825B (de)
FR (1) FR1312325A (de)
GB (1) GB945268A (de)
NL (2) NL133956C (de)
SE (1) SE305667B (de)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3277384A (en) * 1963-11-04 1966-10-04 Hazeltine Research Inc Balanced frequency detector apparatus

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3843928A (en) * 1972-07-28 1974-10-22 Matsushita Electric Ind Co Ltd Fm demodulation system providing noise reduction property

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2227906A (en) * 1938-10-29 1941-01-07 Rca Corp Envelope current device
US2284444A (en) * 1940-08-27 1942-05-26 Bell Telephone Labor Inc Demodulation circuit
US2441957A (en) * 1942-11-13 1948-05-25 Standard Telephones Cables Ltd Demodulator for frequency modulated waves
US2947863A (en) * 1954-03-17 1960-08-02 Hoffman Electronics Corp Linear discriminators or the like

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2484556A (en) * 1946-11-12 1949-10-11 Gen Electric Demodulator for frequency modulated signals

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2227906A (en) * 1938-10-29 1941-01-07 Rca Corp Envelope current device
US2284444A (en) * 1940-08-27 1942-05-26 Bell Telephone Labor Inc Demodulation circuit
US2441957A (en) * 1942-11-13 1948-05-25 Standard Telephones Cables Ltd Demodulator for frequency modulated waves
US2947863A (en) * 1954-03-17 1960-08-02 Hoffman Electronics Corp Linear discriminators or the like

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3277384A (en) * 1963-11-04 1966-10-04 Hazeltine Research Inc Balanced frequency detector apparatus

Also Published As

Publication number Publication date
NL133956C (de)
DE1195825B (de) 1965-07-01
NL273844A (de)
GB945268A (en) 1963-12-23
CH393449A (de) 1965-06-15
SE305667B (de) 1968-11-04
FR1312325A (fr) 1962-12-14

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